Asymmetric membranes have long been used in a variety of purification operations. Permeation selective or permselective membranes preferentially pass certain components of liquid mixtures while retaining other components. In reverse osmosis membranes, a hydrostatic pressure in excess of the equilibrium osmotic pressure of the liquid mixture or solution is applied to the liquid to force more permeable components, usually water, through the membrane in preference to less permeable components, such as salt, contrary to normal osmotic flow.
In preparation of membranes for these applications, a film or hollow fiber is first cast from a casting dope or solution of polymer, solvent and salt; and the solvent and salt are then separated from the polymer to yield a membrane. Effort has long been directed toward improvement of this process to increase the flux and salt rejection of the membranes.